Frequency converters or mixers as they are commonly called, can be classified as single ended, balanced or double balanced. A balanced mixer requires a hybrid device to isolate the local oscillator and the signal powers. A hybrid is a four port microwave circuit that evenly divides the signal that is applied to any of its ports and transfers the signals to two ports while the fourth port is isolated.
Microwave mixers operating above 18GHz have in the past used waveguide hybrid Tees or short slot hybrids. Other balanced waveguide mixers use a coaxial local oscillator feed from a side wall and two diodes across the broad walls of the waveguide as disclosed in U.S. Pat. No. 3,638,126. These devices have a limited bandwidth and are relatively difficult to manufacture and, therefore, expensive. Wide band coaxial or microcircuit hybrids have in the past not been successfully built at frequencies above 18GHz because the required performance characteristics could not be attained with conventional circuits that operate at lower frequencies.